Apparatus and method for separating solids from a solids laden drilling fluid

ABSTRACT

An apparatus for separating solids from solids laden drilling fluid is provided. The apparatus includes a shale shaker and a screen assembly. The shale shaker includes a base and a basket isolated from the base, a flow tray arranged in the basket for directing screened drilling fluid. The screen assembly includes at least one layer of screening material, a screen support having a perimeter with at least one support point within the perimeter, wherein the flow tray includes at least one raised portion to support the screen assembly. In some versions the shale shaker has a basket having side supports and at least one intermediate support arranged between the side supports. The screen assembly includes a screen support supporting at least one layer of screening material.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of GB Patent Application No.1007165.2, filed on Apr. 29, 2010, the entire contents of which arehereby incorporated by reference.

The invention relates to an apparatus and method for separating solidsfrom a solids laden drilling fluid and particularly, but not exclusivelyan apparatus and method for separating solids from a solids ladendrilling mud. The invention also relates to a shale shaker and screenassemblies therefor.

In the drilling of a borehole in the construction of an oil or gas well,a drill bit is arranged on the end of a drill string, which is rotatedto bore the borehole through a formation. A drilling fluid known as“drilling mud” is pumped through the drill string to the drill bit tolubricate the drill bit. The drilling mud is also used to carry thecuttings produced by the drill bit and other solids to the surfacethrough an annulus formed between the drill string and the borehole. Thedensity of the drilling mud is closely controlled to inhibit theborehole from collapse and to ensure that drilling is carried outoptimally. The density of the drilling mud affects the rate ofpenetration of the drill bit. By adjusting the density of the drillingmud, the rate of penetration changes at the possible detriment ofcollapsing the borehole. The drilling mud may also carry lostcirculation materials for sealing porous sections of the borehole. Theacidity of the drilling mud may also be adjusted according to the typeof formation strata being drilled through. The drilling mud containsinter alia expensive synthetic oil-based lubricants and it is normaltherefore to recover and re-use the used drilling mud, but this requiresinter alia the solids to be removed from the drilling mud. This isachieved by processing the drilling mud. The first part of the processis to separate the solids from the solids laden drilling mud. This is atleast partly achieved with a vibratory separator, such as those shaleshakers disclosed in U.S. Pat. No. 5,265,730, WO 96/33792 and WO98/16328. Further processing equipment such as centrifuges andhydrocyclones may be used to further clean the mud of solids. The solidsare covered in contaminates and residues. It is not uncommon to have 30to 100 m³ of drilling fluid in circulation in a borehole.

The resultant solids, known herein as “drill cuttings” are processed toremove substantially all of the residues and contaminates from thesolids. The solids can then be disposed of in a landfill site or bydumping at sea in the environment from which the solids came.Alternatively, the solids may be used as a material in the constructionindustry or have other industrial uses.

Shale shakers generally comprise an open bottomed basket having one opendischarge end and a solid walled feed end. A number of rectangularscreens are arranged over the open bottom of the basket. The screens maybe substantially planar or have a slight crown. The basket is arrangedon springs above a receptor for receiving recovered drilling mud. A skipor ditch is provided beneath the open discharge end of the basket. Amotor is fixed to the basket, which has a drive rotor provided with anoffset clump weight. In use, the motor rotates the rotor and the offsetclump weight, which causes the basket and the screens fixed thereto toshake. Solids laden mud is introduced at the feed end of the basket onto the screens. The shaking motion induces separation of the drillingmud from the solids, the drilling mud passing through the screens andthe solids above the screens. The shaking motion also induces the solidsto move along the screens towards the open discharge end. The recovereddrilling mud is received in the receptor for further processing and thesolids pass over the discharge end of the basket into the ditch or skip.

The rectangular screens may be arranged at an angle to horizontal, suchas a seven degrees incline from the feed end to the discharge end of theshale shaker. The angle may be adjustable. The screens are generallyfixed in the basket and the basket is adjustable to adjust the angle ofthe screens relative to horizontal. The flow of solids laden drillingfluid may form a pool on the inclined screens. The action of thevibratory mechanism induces solids to climb the inclined screens to thedischarge end of the shaker and into the ditch or skip.

Generally, a vibratory mechanism inducing a circular vibration will tendto throw solids from the screen into the air in circular motions, whichis often in shale shakers having a horizontal screen deck. A vibratorymechanism inducing an elliptical motion will induce the solids to movein a direction of the longest chord of the ellipse, which is often usedin shale shakers having an inclined screen deck, such that in usemovement of solids up the inclined screen deck is facilitated. A shaleshaker having a vibratory mechanism inducing a very thin ellipse isknown as a linear shale shaker and induces fast movement of solids alongthe screen, although the screen tends to suffer fast degradation due tosudden deceleration of the solids as they meet the screen.

The screens used in a shale shaker experience high acceleration valuesand heavy loading when processing drilling fluids. The high accelerationvalues are required in order to induce throughput through the screensand in order to convey the solids over the screen for discharge.

The shale shaker vibratory drive apparatus is sized in order to inducean optimal stroke profile based upon the total mass of assemblyincluding the vibratory drive apparatus, basket, screens and solidsladen drilling fluid therein i.e. all of the vibrated components,essentially the sprung mass. The screens are clamped or otherwisefastened to the basket and the screens support the solids laden drillingfluid to be processed.

The motion induced into the assembly is typically a simple harmonicmotion, whereby starting from its lowest point the vibratory drive,basket, screens and solids laden drilling fluids are accelerated upwardstypically in an elliptical motion, the longest chord of the ellipse setat around forty-five degrees towards the front of the basket by highpositive acceleration forces reducing towards mid stroke where theacceleration values reduce to zero then negative deceleration occursreducing the velocity to a minimum at the top of the stroke. This cycleis repeated on the downward stroke showing that high accelerationvalues, (positive and negative) are experienced at each tip of thestroke where the velocity is minimized and the acceleration values arezero at each mid-point of the stroke where the velocities are at theirhighest.

As the fluids are not physically attached to the screens the screenloading varies therefore dependent upon the stroke cycle. During theupward stroke the screen is accelerated into the fluid mass therebyinducing throughput then the screen accelerates away from the fluid masson the downward stroke only for the fluid mass to drop onto the screensagain at the bottom of the stroke due to gravity and the whole cycle isthen repeated typically at approximately 1800 to 2000 rpm.

For optimal performance the screen overall should preferably move as oneentity matching the stroke profile of the basket and any deflectionshould be minimized. Should the screen deflect then this can lead to farhigher acceleration values being induced leading to premature wear anduneven loading across the width of the screen. Given that the motion isalso utilized to convey solids across the screen, any additionaldeflection of the screen will result in uneven conveyance across thescreen width.

Prior art has shown that the screen can span the distance between thescreen clamps on the sides of the basket that are utilized in order toclamp the screen to the basket, but this requires a significant amountof materials in order to provide the rigidity required. The downside ofthis approach is that the screens are generally bigger and heavier, sonot so good for manhandling and may be more expensive to manufacture.

The screens are generally of one of two types: hook-strip; andpre-tensioned.

The hook-strip type of screen comprises several rectangular layers ofmesh in a sandwich, usually comprising one or two layers of fine grademesh and a supporting mesh having larger mesh holes and heavier gaugewire. The layers of mesh are joined at each side edge by a strip whichis in the form of an elongate hook. In use, the elongate hook is hookedon to a tensioning device arranged along each side of a shale shaker.The shale shaker further comprises a crowned set of supporting members,which run along the length of the basket of the shaker, over which thelayers of mesh are tensioned. An example of this type of screen isdisclosed in GB-A-1,526,663. The supporting mesh may be provided with orreplaced by a panel having apertures therein.

The pre-tensioned type of screen comprises several rectangular layers ofmesh, usually comprising one or two layers of fine grade mesh and asupporting mesh having larger mesh holes and heavier gauge wire. Thelayers of mesh are pre-tensioned on a rigid support comprising arectangular angle iron frame and adhered thereto. The screen is theninserted into C-channel rails arranged in a basket of a shale shaker. Anexample of this type of screen is disclosed in GB-A-1,578,948 and anexample of a shale shaker suitable for receiving the pre-tensioned typescreens is disclosed in GB-A-2,176,424.

WO 2004/035234 discloses a screen assembly for a shale shaker, thescreen assembly comprising a panel and a support structure, the panelhas an area provided with a multiplicity of apertures and at least onelayer of screening material arranged over the multiplicity of apertures.The apertures have downwardly projecting tabs to provide rigidity to thepanel. Tabs of adjacent apertures form panel ribs. The support structurecomprises a plurality of transverse support ribs. Transverse panel ribsfit over the transverse support ribs. The panel is removable from thesupport structure.

WO 03/013690 and WO 2004/069374 disclose a screen assembly comprising ascreen element and a separate support. The screen element comprising amesh panel and side inclined support members having a downwardlyinclined face. The support comprises a rigid frame having side inclinedsurfaces at each side. When in use, the screen assembly is slid intorails of a shale shaker. A pneumatic seal arranged in the rails isactivated to push the side inclined support members on to the inclinedsurfaces of the rigid frame to tension the mesh panel over the support.The screen element may comprise a light weight flexible apertured plate.

A problem associated with shale shakers is that screens used thereintend to blind, especially when the solids are gummy, such as clay, or ofa size close to the size of the mesh size of the screen. The latter typeof blinding is known as near-sized particle blinding. A number ofsolutions have been proposed to tackle this problem, such as disclosedin GB-A-1,526,663 in which a screen assembly using two layers ofscreening material in a sandwich and allowing the layers of screeningmaterial to move independently to dislodge any near sized particleslodged in one of the screens. WO 01/76720 relating to the rotatable drumseparator also discloses using air nozzles external to the perforatedrum to provide positive pressure air through the perforate drum in thedry zone to ensure that the perforations do not clog with solids.

It is advantageous to use fine-meshed filters to filter very smallparticles, for example of a size in the range of 50-200μ or more,without the filtering device clogging up with the small particles.However, it is the fine-meshed filters in particular that are prone tosuch undesired clogging.

It is also advantageous to provide a separator which operates at lownoise levels to comply with health and safety legislation. It is alsoadvantageous to have a simple reliable separator to inhibit down-timefor maintenance and repair.

It is preferable in certain circumstances to retain particles, forexample of a particle size in the range of 50-60μ or larger, by means ofa filter.

In the drilling of an oil or gas well, cracks may exist in the wall ofthe wellbore. Such cracks may propagate, which could cause structuralproblems in the wall of the wellbore and/or allow drilling fluids toescape therethrough into the formation. Furthermore, if substantialamounts of drilling fluids are lost, the pressure in the drilling fluidin the wellbore may drop, which could cause collapse of the wellbore.Accordingly, Wellbore Strengthening Materials may be added to circulateddrilling fluid. The Wellbore Strengthening Materials comprise sizedparticles. When the drilling fluid is circulated around the wellborewall with cracks therein, the sized particles wedge themselves in thecracks, which reduces the likelihood of the cracks propagating. It isbeneficial to recover these sized particles and reuse them in circulateddrilling mud. Shale shakers have thus been modified to size solids insolids laden drilling fluid. Such a shale shaker is disclosed in U.S.Ser. No. 12/490,492 US2010/0089652. A range of sizes of solids can beextracted using such a shale shaker and recirculated as WellboreStrengthening Material in fresh drilling fluid.

In order to expedite screening of solids laden drilling mud, energy isrequired to be imparted to the solids laden drilling at the juncturewhere the filtering takes place, i.e. where the solids laden drillingmud meets the screen.

Prior art also shows that the basket configuration may be such thatadditional support is offered to the screen between sides of the basket,GB-A-2,206,501 thereby reducing the span of the screen hence reducingthe rigidity required of the screen such that the screen can be madesmaller and lighter.

US-A-2007/0187303 Bailey and Fisher, discloses a screen assembly forseparating solids from solids laden drilling fluid, the screen assemblycomprising a screen element and a support frame insertable into a clampapparatus of a shale shaker. The support frame has a plurality ofsupport elements. The screen element comprises mesh panels havinglongitudinal cranked edge portions which, in use, are clamped betweeninclined surfaces of the support frame to fix the mesh panels over thesupport elements therebetween. It should be noted that, in use, screenedfluid flows through the screen element and support element.

GB-A-2,425,743 Burnett, discloses a screen assembly for separatingsolids from solids laden drilling fluid, the screen assembly shown inFIG. 1 thereof comprises a screen panel for receiving screening materialand a screen support insertable into a clamp apparatus of a shaleshaker. The screen panel comprises longitudinal inverted T-rails whichare pulled down on to the screen support by action of being clamped inthe clamp apparatus of the shale shaker. The screen assembly shown inFIG. 2 of GB-A-2,425,743 comprises a screen panel with screeningmaterial thereon and a separate screen support having crown ribsinsertable into a clamp apparatus of a shale shaker. The screen panelclamped on to the screen support in the clamping apparatus of the shaleshaker. It should be noted that, in use, screened fluid flows throughthe screen element and support element.

The present invention offers increased support to the screens byminimizing the span between supports and the use of tapered surfacesencourages the screen to wedge itself into the support structure. Thisin conjunction with the increased contact area between the screen andthe support arrangement dictate that the friction forces to be overcomewhen lifting the screen from the support arrangement are higher thanthat required for a simple point contact arrangement.

For the simple point contact arrangement the reaction forces acting onthe screen and support structure are vertical only such that support isgiven during the upward stroke of the basket only.

The tapered surfaces proposed dictate that the reaction forces areangled perpendicular to the angled faces and that equivalent loads canbe calculated comprising of a vertical component and a horizontalcomponent. The horizontal component reacts against the screen and is anadditional force to be overcome as the screen deflects under its ownweight during the downward stroke of the basket. Maximizing this forcetherefore will help minimizing screen deflection during the downwardstroke.

According to the present invention, there is provided an apparatus forseparating solids from solids laden drilling fluid, the apparatuscomprising a shale shaker and a screen assembly, the shale shakercomprising a flow tray for directing screened drilling fluid, the screenassembly comprising at least one layer of screening material, a supportand a perimeter with at least one support point within the perimeter,characterized in that the flow tray comprises at least one raisedportion to support the screen assembly. Thus the flow tray is preferablyused as a Structural component to increase the rigidity of the screenassembly.

Preferably, the flow tray is arranged at an angle between horizontal andtwenty five degrees from horizontal and most preferably, between fiveand fifteen degrees, advantageously, twelve degrees. Advantageously, theflow tray directs screened drilling fluid to the feed end of the basketwhereupon preferably, the screened drilling fluid is directed into atleast one duct and preferably into a duct of a flow distributorapparatus. Preferably, the screen assembly lies in a first plane and theflow tray lies in a second plane, the screen assembly in the first planediverging from the flow tray in the second plane. Advantageously, thefirst and second planes diverge at an angle of between one and tendegrees. Preferably, the flow tray comprises a series of raised portionsconfigured to support the screen assembly lying at the diverging angleto the flow tray.

Preferably, the at least one raised portion comprises at least one wearstrip. Advantageously, the flow tray is made from a composite material.Advantageously, the at least one raised portion defines flow channelsfor directing the screened drilling fluid along the flow tray.Preferably, the raised portion forms a longitudinal wall spanning atleast a major portion and advantageously the entire length of the flowtray. Preferably, the flow tray is substantially the same length as thescreen deck on which the screen assembly or screen assemblies lie inuse. Alternatively, the flow tray has a length which is a major portionof the length of the screen deck. Preferably, the flow channels aretapered to facilitate flow of screened drilling mud. Preferably, taperedin depth, the flow channels have a high underside at a first end and alow height at the second end at which the screened drilling fluid isdischarged. Considerable longitudinal forces are induced in the screenassembly by the vibratory mechanism in use. The tapered portionspreferably fit over corresponding tapered portions, thus providing alarge contact area. The large contact area provides a high degree ofsurface tension and thus inhibits movement of the screen assemblyrelative to the screen deck and hence the basket, whilst still allowingeasy insertion of the screen assemblies when replacing the screenassemblies.

Advantageously, the shale shaker comprises a basket, the flow tray fixedto the basket. Preferably, the basket comprises sides, the flow trayfixed to the sides. Advantageously, the sides each have a rail, the flowtray supported on the rail. Preferably, the flow tray is adhered,riveted, screwed, glued, welded or otherwise fixed to the rail and/orsides. Alternatively, the flow tray is removable from the basket,preferably slidably removable.

Preferably, the flow tray comprises a plurality of contours, at leastone of the contours forms the at least one of the raised portion.

Advantageously, the at least one raised portion is a plurality of raisedportions. The more raised portions, the better the support for thescreen assembly, although the flow channels need to be sized to removethe throughput of screened drilling mud, which can be in the order of athousand gallons per minute.

Preferably, the screen support comprises a tapered surface.Advantageously, the at least one raised portion comprises a taperedsupport surface to receive the tapered surface of the screen support.Preferably, the tapered surface is formed from a sheet material, whichmay be sheet metal folded to form a tapered surface. Preferably, thesheet material has openings therein to allow screened drilling mud toflow through the screening material.

U.S. Pat. No. 6,454,099 B1 Adams, discloses screen assemblies for ashale shaker for separating solids from solids laden drilling fluid, thescreen assemblies having hookstrips with support strips and screeningmaterial arranged therebetween. The screening material has raisedportions therein.

U.S. Pat. No. 6,302,276 B1 Seyffert, discloses a screen assembly for ashale shaker for separating solids from solids laden drilling fluid, thescreen assembly having a strip support with strips arranged across theshaker and having raised portions to support screening material havingundulations therein, the undulations running parallel with the sides ofthe screen assembly. Also disclosed in a screen assembly comprisingreplaceable panels.

The present invention also provides an apparatus for separating solidsfrom solids laden drilling fluid, the apparatus comprising a shaleshaker and a screen assembly, the shale shaker comprising a baskethaving side supports and at least one intermediate support arrangedbetween the side supports, the screen assembly comprising a screensupport supporting at least one layer of screening material, the screensupport having at least two sides to be supported by the side supportsand at least one support member between the sides for engaging with theat least one intermediate support, characterized in that the screensupport comprises a tapered surface and the at least one intermediatesupport has a tapered support surface, such that, in use, the taperedsurface and the tapered support surface abut. Preferably, thisarrangement inhibits lateral and tangential movement of said screenassembly in said basket. Advantageously, the tapered surface or taperedsurfaces are substantially planar, although they may be curved; compriseridges; and/or have a roughened surface. Preferably, the further taperedsurface is of the same magnitude as the angle from the horizontal as thetapered surface. Advantageously, the further tapered surface is of adifferent magnitude as the angle from the horizontal as the taperedsurface. Preferably, the angles of the tapered support surface andfurther tapered support surface are the same or substantially the sameto obtain a large surface area of contact therebetween.

Preferably, the screen support comprises a further tapered surface at anopposing angle to the tapered surface and the at least one intermediatesupport has a further tapered support surface at an opposing angle tothe tapered support surface, such that, in use, the tapered surface andfurther tapered surface abut the tapered support surface and the furthertapered support surface. Advantageously, the tapered surface and thefurther tapered surface of the screen support diverge from the screensupport, preferably diverging at an acute angle, although they may be atan obtuse angle. Preferably, the tapered support surface and the furthertapered support surface of said intermediate support converge from theintermediate support, advantageously at a converging acute angle,although they may be at an obtuse angle.

The present invention also provides a screen assembly comprising atleast one layer of screening material, a screen support and a perimeterwith a plurality of support points within the perimeter characterized inthat at least one of the support points comprises a tapered surface.

Preferably, the tapered surface lies in a plane which is between ten andeighty degrees from horizontal. Advantageously, the tapered surface liesin a plane which is between forty and seventy degrees from horizontal.

Advantageously, the screen assembly comprises a further tapered surfaceat an opposing angle to the tapered surface and the at least oneintermediate support has a further tapered support surface at anopposing angle to the tapered support surface, such that, in use, thetapered surface and further tapered surface abut the tapered supportsurface and the further tapered support surface. Preferably, thisarrangement inhibits lateral movement of said screen assembly in saidbasket. Preferably, the further tapered surface is of the same magnitudeas the angle from the horizontal as the tapered surface. Advantageously,the further tapered surface is of a different magnitude as the anglefrom the horizontal as the tapered surface.

The present invention also provides a shale shaker comprising a base, abasket isolated from the base, a vibratory apparatus for vibrating thebasket and screen deck in the basket and a flow tray for directingscreened solids laden drilling mud characterized in that the that theflow tray comprises at least one raised portion to support the screenassembly.

Preferably, the basket further comprises a second screen deck, the flowtray directing solids laden drilling fluid thereon.

The present invention also provides a method of using the apparatus ofthe invention, the method comprising the step of placing the screenassembly in the basket of the shale shaker, flowing solids ladendrilling fluid on to the screen assembly, at least a proportion of thesolids flowing over the screen assembly and at least a proportion ofsaid drilling fluid passing through said screen assembly.

For a better understanding of the present invention, reference will nowbe made, by way of example, to the accompanying drawings, in which:

FIG. 1 shows a perspective view of an apparatus for separating solidsfrom solids laden drilling mud;

FIG. 2A is a side cross-sectional schematic view of an apparatus forseparating and sizing solids from solids laden drilling mud, theapparatus comprising a base and a basket floating thereon;

FIG. 2B is an end view of the basket shown in FIG. 2A showing screendecks and screen assemblies therein;

FIG. 3 is a schematic end view, partly in section of a screen deck, aflow tray and a screen assembly;

FIG. 3A is a top plan view of the flow tray shown in FIG. 3;

FIG. 4 is a schematic end view, partly in section of a screen deck, aflow tray and a screen assembly in accordance with the invention;

FIG. 4A is a top plan view of the flow tray shown in FIG. 4;

FIG. 5 is a schematic end view, partly in section of a screen deck, aflow tray and a screen assembly in accordance with the invention; and

FIG. 6 is a schematic end view, partly in section of the flow tray shownin FIG. 5 spaced from the screen assembly shown in FIG. 5;

FIG. 6A is a top plan view of the flow tray shown in FIG. 5;

FIG. 6B is a perspective view of the screen assembly shown in FIG. 5;

FIG. 7 is a schematic end view in cross section of the screen assemblyshown in FIG. 5, spaced from a flow tray in accordance with the presentinvention; and

FIG. 8 is a schematic end view of a screen assembly seated on a flowtray in accordance with the present invention.

FIG. 1 shows an apparatus for separating solids from a solids ladendrilling mud, generally referred to as a shale shaker and identifiedherein by reference H. The shale shaker H comprises a base D having anopen bottom R arranged above a collection receptacle (not shown) forreceiving screen drilling mud. A basket B is arranged on springs C onthe base D. A vibratory apparatus E is arranged on top of the basket B.The vibratory apparatus E comprises an electric or hydraulic motor (notshown) rotating offset clump weights hidden within casing S, whichinduce a motion in the basket B. An upper, upper middle, lower middleand lower screen assemblies A1, A2, A3, A4 are arranged in the basket Band fixed thereto in rails (not shown) so that the motion induced in thebasket is transferred to the screen assemblies A1, A2, A3, A4. Solidsladen drilling fluid is fed on to the screen assemblies A1-A4 from afeed chamber F at a feed end of the apparatus. The motion induced in thescreen assemblies A1-A4 facilitates separation of solids from drillingmud. Screened drilling mud passes through the screen assemblies into thecollection receptacle (not shown) and solids climb along the screenassemblies A1-A4 to a discharge end P of the shale shaker and into askip, ditch or other cuttings transfer apparatus (not shown).

FIGS. 2A and 2B show a shale shaker 10 having a base 20 and a basket 30arranged thereon on springs (not shown). The basket 30 comprising ascalping deck 11, an upper primary deck 12 and a lower primary deck 13.The upper primary deck 12 has a left-hand side 12 a and a right-handside 12 b. The lower primary deck has a left-hand side 13 a and aright-hand side 13 b. Solids laden drilling fluid is introduced to afeed end of the shale shaker 14 from a feeder (not shown) on to ascalping screen 15 arranged in C-shaped channels 16 of the scalping deck11. An expandable pneumatic bladder 17 is arranged in a top part of theC-shaped channels 16 to clamp the scalping screen 15 therein.Alternatively, a wedge may be used to secure the scalping screen 15 inthe C-shaped channels 16. The scalping screen 15 comprises a screen 18having relatively large openings for inhibiting large particles frompassing though on to the primary decks, but allowing some solids anddrilling mud therethrough. The scalping deck 11 and the screen 15thereon is arranged at an uphill tilt of approximately two degrees fromhorizontal, although the scalping screen 15 and scalping deck 11 may bearranged horizontally, slightly downhill or at a slightly greater uphillangle. Vibratory apparatus 16 a fixed to the basket 30 induces motiontherein. The motion facilitates separation of large solids from thesolids laden drilling mud and induces movement in the large solids alongthe scalping screen 15 from the feed end 14 to the discharge end 19 ofthe shale shaker. The large solids may be caught in a ditch or on aconveying means and further processed or used in other operations. Thesolids laden drilling fluid which passes through the scalping screen 15falls on to a flow tray 21 which directs the solids laden drilling fluidto the feed end 14 of screen assemblies 22 a and 22 b in the upperprimary deck 12. A weir 23 is arranged at the feed end of the primarydeck 12 to retain the solids laden drilling fluid. If the level of thesolids laden drilling fluid rises beyond the height of the weir 23,solids laden drilling fluid passes thereover into a duct 24 and on toscreen assemblies 25 a and 25 b in the lower primary screen deck 13. Thescreen assemblies 22 a, 22 b and 25 a and 25 b are preferably of thesame type and have the same screen mesh thereon.

A gate valve 26 in the form of a slideable tray, is in a closedposition, closing off a duct 29, to run the shale shaker in a parallelmode. Screened drilling mud falls through the screen assemblies 22 a and22 b in the upper screen deck on to a flow tray 27 and over the closedgate valve 26 and into a duct 28 which runs parallel to the duct 24.However, duct 28 leads to the bottom of the basket and directly into thecollection receptacle (not shown) therebelow. Solids fall off thedischarge end 31 of upper screen deck 12 and discharge end 32 of thelower screen deck 13 and into a skip or other conveying apparatus forconveying the solids for further processing or re-use.

The gate valve 26 may be retracted to allow drilling mud screened by thescreen assemblies 22 a and 22 b in the upper screen deck 12 to befurther screened by screen assemblies 25 a and 25 b on the lower screendeck 13. The shale shaker thus runs in a series mode. In this situation,it is preferable to use a finer screen mesh in the screen assemblies 25a and 25 b than the screen mesh used in screen assemblies 22 a and 22 b.Drilling mud screened by the screen assemblies 22 a and 22 b in theupper screen deck 12 flows in the flow tray 27 and into a duct 29, whichdirects the screened drilling mud on to the feed end of the screenassemblies 25 a and 25 b in the lower screen deck 13. Sized solids falloff the discharge end 31 of upper screen deck 12 into a conveyor (notshown) to be conveyed and mixed into a fresh batch of drilling mud forre-circulation. These sized solids are used to block cracks in theformation as hereinbefore described. Solids discharges from thedischarge end 32 of the lower screen deck 13 are conveyed in a separateconveyor or added to a skip for further processing or used for otherpurposes.

Referring to FIGS. 3 and 3A, there is shown a screen assembly 100 and aflow tray 101. The flow tray 101 comprises a recessed zone 102, a closedend 103, an open end 104 and a pair of sides 105 and 106. The sides 105and 106 comprise flanged portions 107 and 108, which flanged portions107 and 108 are slid into C-shaped channels 109 and 110 in basket 30 andfixed therein. Preferably, the flanged portions 107 and 108 are glued orotherwise adhered to the C-shaped channels 109 and 110, but may beslidably removable from the channels and fixed with the screen assembly100 by expandable pneumatic hose seal 111. The recessed zone 102 mayhave a substantially horizontal planar base, which when fixed in saidC-shaped channels 109 and 110, assumes a slope equal to the slope of thescreen assembly 100, which is preferably seven degrees. The tray alsohas an opening 112 therein. The gate valve 26 of the shale shaker 10selectively prevents and allows screened drilling mud through theopening 112.

FIG. 4 shows a screen assembly 200 and a flow tray 201 in accordancewith the present invention. The flow tray 201 comprises three recessedzones 202 a, 202 b and 202 c, a closed end 203, an open end 204 and apair of sides 205 and 206. The sides 205 and 206 comprise flangedportions 207 and 208, which flanged portions 207 and 208 are slid intoC-shaped channels 109 and 110 in basket 30 and fixed therein.Preferably, the flanged portions 207 and 208 are glued or otherwiseadhered to the C-shaped channels 109 and 110, but may be slidablyremovable from the channels and fixed with the screen assembly 200 byexpandable pneumatic hose seal 111. The recessed zones 202 a, 202 b and202 c may have a substantially horizontal planar base, which when fixedin said C-shaped channels 109 and 110, assumes a slope equal to theslope of the screen assembly 200, which is preferably seven degrees. Thetray also has an opening 212 therein. The gate valve 26 of the shaleshaker 10 selectively prevents and allows screened drilling mud throughthe opening 212. A pair of intermediate supports 213 and 214 bind therecessed zones 202 a, 202 b and 202 c. The supports 213 and 214 compriseupstands 215 and 216 spanning the length of the flow tray 201. Theupstands each have a T-connector 217 and 218 fixed thereto or integraltherewith, with a wear strip arranged thereon. The wear strip may be anyhard wearing material such as HDPE. A top of the wear strips 219 and 220lie slightly above the top of the flanged portion 207 and 208 of theflow tray, preferably by 2 or 3 mm at the center. The screen assembly200 sits on top of the wear strips 219 and 220 and is held thereon byinflation of the inflatable pneumatic hose 111 in the C-shaped channels109 and 110. Structural rigidity of said flow tray is thus used insupporting the screen assembly 200.

FIGS. 5 to 6B show a screen assembly 300 and a flow tray 301 inaccordance with the present invention. The flow tray 301 comprises sixrecessed zones 302 a, 302 b 30 302 c, 302 d, 302 e and 302 f, a closedend 303, an open end 304 and a pair of sides 305 and 306. The sides 305and 306 comprise flanged portions 307 and 308, which flanged portions307 and 308 are slid into C-shaped channels 109 and 110 in basket 30 andfixed therein. Preferably, the flanged portions 307 and 308 are glued orotherwise adhered to the C-shaped channels 109 and 110, but may beslidably removable from the channels and fixed with the screen assembly300 by expandable pneumatic hose seal 111. The recessed zones 302 a to302 f may have a substantially horizontal planar base, which when fixedin said C-shaped channels 109 and 110, assumes a slope from thedischarge end 31 to the feed end 14 of the shale shaker equal to theslope of the screen assembly 200, which is preferably seven degrees. Thetray also has opening 312 a, 312 b, 312 c and 312 d therein. The gatevalve 26 of the shale shaker 10 selectively prevents and allows screeneddrilling mud through the opening 312 a to 312 d. Five intermediatesupports 313 a, 313 b, 313 c, 313 d and 313 e bind the recessed zones302 a to 302 e. Only support 313 e will be described in detail herein,although it should be noted that all the structural support members 313a to 313 e are as described for structural support member 313 e. Thesupport 313 comprises an upstand 317 spanning the length of the flowtray 301. The upstand 317 has a strip 315 fixed or integral with theupstand 317. The strip 315 has two sides each having a tapered portion318 and 319. The taper of each tapered portion 318, 319 is preferably atan acute angle from the plane of the flow tray 301, preferably betweenten and eighty degrees, advantageously, forty-five to seventy degreesand most preferably sixty to seventy degrees from a horizontal plane.Each tapered portion 318 and 319 has there along a T-shaped member 320and 321 fixed thereto or integral therewith, with a wear strip 322 and323 arranged thereon. The tapered portions 318 and 319 each taper at aconverging angle from the flow tray 301. The wear strips 322 and 323 maymade from a hard wearing material such as HDPE. A top 325 of the strip315 lies slightly above the top of the flanged portion 307 and 308 ofthe flow tray, preferably by 2 or 3 mm at the center support 313 c, 1 to2 mm on the adjacent supports 313 b and 313 d and 0.5 to 1 mm on theouter supports 313 a and 313 e. The wear strips 322 and 323 have achamfered portion 324, and the top 325 of the strip 315 has a slightcrown. The chamfered portions 324 provide a smooth transition betweenthe top 325 and the wear strips 322 and 323.

The screen assembly 300 comprises a screen support 340 comprising aperforate plate 351, such as a perforate plate of metal or perforateplastics or perforate composite material. The perforations in theperforate plate 351 may be rectangular, as shown and which is preferredor any other suitable shape, such as octagonal, hexagonal, triangular,square, round. At least one layer of screening material 299 is adheredor otherwise attached to a top surface of the perforate plate 351. Theat least one layer of screening material 351 may be two layers ofscreening material of equal mesh size and may be supported on asupporting layer of screening material having a larger mesh size andlarge gauge wires.

The screen support 340 comprises outer tubular frame members 352 and 353along each side of the screen support. Each tubular frame member 352 and353 is of hollow rectangular cross-section. Structural support members350 a to 350 e are welded or otherwise attached or integral with theperforate plate 351. Only structural support member 350 a will bedescribed in detail herein, although it should be noted that all thestructural support members 350 a to 350 e are as described forstructural support member 350 a. Structural support member 350 a issymmetrical having a first side 354 and a second side 355 which aremirror images of each other. The first side 354 has an outer taperedportion 356 arranged in a plane at approximately sixty degrees fromhorizontal. A top edge of the outer tapered portion 356 is welded orotherwise attached to the perforate plate 351. An inner tapered portion357 is linked to the other side 355 with a curved portion 358. The innertapered portion 357 of side 354 and the inner tapered portion 357 a ofside 355 diverge from the perforate plate 351. The curved portion 358 isshaped such that, in use, it sits over the crowned top 325 and chamferedportions 324 of the wear strips 322 of each support 313 a to 313 e ofthe flow tray 301. The curved portion 358 is not fixed to the perforateplate 351 and stands clear with a gap therebetween. The lower part 359lies in a plane of approximately sixty-five degrees from horizontal toconform to an outer face of the wear strip 324 on the tapered portion318 of each support 313 a to 313 e. The structural support members 350 ato 350 e further comprise a curved portion 358 linking the two sides 354and 355. The structural support members 350 a to 350 e have a pluralityof openings 360 therein to allow fluid to flow therethrough on to theflow tray 301. The structural support members 350 a to 350 e may be madefrom sheet metal such as galvanized steel and may be formed by foldingthe sheet metal. The openings 360 may be punched or laser cut in thesheet metal prior to folding. Alternatively, the structural supportmembers 350 a to 350 e may be formed from a composite material such as afiber reinforced material such as KEVLAR™.

In use, the screen assembly 300 is inserted into the C-shaped channels109 and 110 above the flow tray 301. The inner tapered portions 357 ofthe structural support members 350 a to 350 e slide along theirrespective wear strips 322 a to 322 e and 323 a to 323 e duringinsertion and then sit thereon. The screen assembly 300 is clamped inplace by inflation of the inflatable pneumatic hose 111 in the C-shapedchannels 109 and 110. Inflation of the pneumatic hose 111 pushes thesides of the screen assembly 300 on to the flanged portions 307 and 308of the flow tray 301 and the inner tapered portions 357 of thestructural support members 350 a to 350 e slide down their respectivewear strips 322 and 323 to 323 e to achieve a tight fit. Gaps betweenthe top 325 a to 325 e and the screen support are likely due totolerances in the relative size and location of the structural supportmembers 350 a to 350 e and the supports 313 a to 313 e of the flow tray301. However, the abutment of the inner tapered portions 357 of thestructural support members 350 a to 350 e and their respective wearstrips 322 and 323 provide a large contact area for supporting thescreen assembly 300. Furthermore, the screen assembly 300 is inhibitedfrom lateral movement in the basket 30 of the shale shaker.

FIG. 7 shows the screen assembly 300 shown in Figure FIGS. 5 to 6B witha flow tray 401. The flow tray 401 comprises a comprises six recessedzones 402 a, 402 b 402 c, 402 d, 402 e and 402 f to facilitate flow ofdrilling fluid therealong, a far end 403, an open end 404 and a pair ofsides 405 and 406. The sides 405 and 406 comprise flanged portions 407and 408, which flanged portions 407 and 408 are slid into C-shapedchannels 109 and 110 in basket 30 and fixed therein. Preferably, theflanged portions 307 and 308 are glued or otherwise adhered to theC-shaped channels 109 and 110 and may by riveted or mechanically fixed,but may be slidably removable from the channels and fixed with thescreen assembly 300 by expandable pneumatic hose seal 111. The recessedzones 402 a to 402 f have an undulating profile which taper in depthfrom the far end 403 to the open end 404, which assumes a slope from thedischarge end 31 to the feed end 14 of the shale shaker greater than theslope of the screen assembly 300. The slope of the screen assembly maybe adjustable between ten degrees uphill and one degree downhill, but ispreferably fixed at seven degrees uphill. The flow tray 401 also has anopening (not shown) therein. The gate valve 26 of the shale shaker 10selectively prevents and allows screened drilling mud through theopening. Five intermediate supports 413 a, 413 b, 413 c, 413 d and 413 ebind the recessed zones 402 a to 402 e. The flow tray 401 is preferablyformed in a mold with composite material, such as KEVLAR™. Only support413 e will be described in detail, although it should be noted that allthe structural support members 413 a to 413 e are as described forstructural support member 413 e. The support 413 comprises an upstand417 molded into the composite flow tray 401 spanning the length of theflow tray 401. The upstand 417 has a strip 415 fixed or integral withthe upstand 417. The strip 415 has two sides each having a taperedportion 418 and 419. The taper of each tapered portion 418, 419 ispreferably between ten and eighty degrees, and preferably forty-five toseventy degrees and most preferably sixty to seventy degrees fromhorizontal. Each tapered portion 418 and 419 has therealong a T-shapedmember 420 and 421 fixed thereto or integral therewith, with a wearstrip 422 and 423 arranged thereon. The wear strips 422 and 423 may madefrom a hard wearing material such as HDPE. A top 425 of the strip 415lies slightly above the top of the flanged portion 407 and 408 of theflow tray, preferably by 1 to 3 mm. The wear strips 422 and 423 have achamfered portion 424, and the top 425 of the strip 415 has a slightcrown. The chamfered portions 424 provide a smooth transition betweenthe top 425 and the wear strips 422 and 423.

FIG. 8 shows a screen assembly 500 and a flow tray 501. The flow tray501 comprises six recessed zones 502 a, 502 b 502 c, 502 d, 502 e and502 f to facilitate flow of drilling fluid therealong, a far end 503, anopen end 504 and a pair of sides 505 and 506. The sides 505 and 506comprise flanged portions 507 and 508, which flanged portions 507 and508 are slid into C-shaped channels and in basket and fixed therein, asbefore described. Preferably, the flanged portions 507 and 508 are gluedor otherwise adhered to the C-shaped channels and may be riveted ormechanically fixed, but may be slidably removable from the channels andfixed with the screen assembly 500 by expandable pneumatic hose seal.The recessed zones 502 a to 502 f have an undulating profile which taperin depth from the far end 503 to the open end 504, which assumes a slopefrom the discharge end 31 to the feed end 14 of the shale shaker greaterthan the slope of the screen assembly 500. The slope of the screenassembly may be adjustable between ten degrees uphill and one degreedownhill, but is preferably fixed at seven degrees uphill. The flow tray501 also has an opening (not shown) therein. The gate valve 26 of theshale shaker 10 selectively prevents and allows screened drilling mudthrough the opening. The screen assembly comprises at least one layer ofscreening material 509 on a support structure 510. The support structurecomprises a frame 511. The flow tray 501 is preferably formed in a moldwith a composite material, such as KEVLAR™.

In use, the screen assembly 500 is inserted into the C-shaped channels109 and 110 above the flow tray 501. Five support ribs 512 a to 512 eeach having a wear strip thereon, are welded, glued or otherwiseattached to flat topped crests 513 a to 513 e of the flow tray. A topsurface of the support ribs 512 a to 512 e preferably lie in a planewhich is between 1 and 3 millimeters above from the plane of the top ofthe flanges 507 and 508. The support structure 510 of the screenassembly 500 sits on five support ribs 512 a to 512 e. The screenassembly 500 is clamped in place by inflation of the inflatablepneumatic hose 111 in the C-shaped channels 109 and 110. Inflation ofthe pneumatic hose 111 pushes the sides of the screen assembly 500 on tothe flanged portions 507 and 508 of the flow tray 501.

What is claimed is:
 1. An apparatus for separating solids from solidsladen drilling fluid, the apparatus comprising: a shale shaker and ascreen assembly, the shale shaker comprising a basket and a vibratorymechanism for vibrating the basket, the basket having side supports andat least one longitudinal intermediate support arranged between saidside supports, the screen assembly comprising a screen supportsupporting at least one layer of screening material, the screen supporthaving a length with at least two side portions substantially parallelto the length of the screen support and at least one longitudinalsupport member substantially parallel to the length of the screensupport between said side portions for engaging with said longitudinalintermediate supports and the at least two side portions supported bysaid side supports; wherein the at least one longitudinal support memberis connected to a bottom surface of the screen support, the at least onelongitudinal support member having a tapered surface and a furthertapered surface at an opposing angle to said tapered surface along thelength of the screen support, wherein the tapered surface and thefurther tapered surface are connected by a curved portion, and wherein aportion of the tapered surface and a portion of the further taperedsurface extend beyond the bottom surface of the screen support; andwherein the at least one longitudinal intermediate support comprises atapered support surface, a further tapered support surface at anopposing angle to said tapered support surface, and a strip comprising acrowned top therebetween, wherein the tapered surface and the furthertapered surface correspond to the tapered support surface and thefurther tapered support surface of the at least one longitudinalintermediate support for surface to surface abutting engagementtherewith along the length of the screen support, and wherein the curvedportion is shaped to fit over the crowned top of the at least onelongitudinal intermediate support.
 2. The apparatus as claimed in claim1, wherein said tapered surface and said further tapered surface of theat least one longitudinal support member diverges from the screensupport.
 3. The apparatus as claimed in claim 1, wherein said taperedsupport surface and said further tapered support surface of the at leastone longitudinal intermediate support converges from said screensupport.
 4. The apparatus of claim 1, wherein the screen assemblyfurther comprises: a perimeter with a plurality of support points withinsaid perimeter; wherein at least one of said support points comprises atapered surface and a further tapered surface at an opposing angle tosaid tapered surface.
 5. The apparatus as claimed in claim 4, whereinsaid tapered surface and said further tapered surface diverge from saidscreen support.
 6. The apparatus as claimed in claim 4, wherein thetapered surface lies in a plane which is between ten and eighty degreesfrom horizontal.
 7. The apparatus as claimed in claim 4, wherein thetapered surface lies in a plane which is between forty and seventydegrees from horizontal.
 8. A method of using the apparatus as claimedin claim 1, the method comprising: placing the screen assembly in thebasket of the shale shaker, wherein the tapered surface and the furthertapered surface at an opposing angle to the tapered surface abut thecorresponding tapered support surface and the further tapered supportsurface at an opposing angle to the tapered support surface; flowing thesolids laden drilling fluid on to the screen assembly with at least aportion of the solids flowing over the screen assembly and at least aportion of said drilling fluid passing through said screen assembly. 9.The apparatus as claimed in claim 1, wherein the at least oneintermediate support comprises at least one raised portion of a flowtray.
 10. The apparatus as claimed in claim 9, wherein the basketfurther comprises a second screen deck, the flow tray directing thesolids laden drilling fluid thereon.
 11. The screen assembly as claimedin claim 4, wherein said tapered surface and said further taperedsurface are formed from a sheet material.
 12. A screen assembly for ashale shaker, the screen assembly comprising: a screen supportsupporting at least one layer of screening material, the screen supporthaving a length with two sides substantially parallel to the length ofthe screen support; and a multiplicity of longitudinal structuralsupport members connected to a bottom surface of the screen supportbetween said two sides and disposed substantially parallel to the lengthof the screen support, wherein at least one of the longitudinalstructural support members comprises a tapered surface and a furthertapered surface at an opposing angle to said tapered surface along thelength of the screen support, wherein the tapered surface and thefurther tapered surface are linked by a curved portion, wherein aportion of the tapered surface and a portion of the further taperedsurface extend beyond the bottom surface of the screen support, whereinthe tapered surface and the further tapered surface correspond to atapered support surface and a further tapered support surface of atleast one longitudinal intermediate support for surface to surfaceabutting engagement therewith along the length of the screen support,and wherein the curved portion is shaped to fit over a crowned top ofthe at least one longitudinal intermediate support.
 13. The screenassembly as claimed in claim 12, wherein the screen support is a planarpanel with openings therein.
 14. The screen assembly as claimed in claim12, wherein said further tapered surface diverges from said taperedsurface.
 15. The screen assembly as claimed in claim 12, wherein saidfurther tapered surface is at an opposing angle to the tapered surface.16. The screen assembly as claimed in claim 12, wherein the taperedsurface lies in a plane which is between ten and eighty degrees fromhorizontal.
 17. The screen assembly as claimed in claim 12, wherein thetapered surface lies in a plane which is between forty and seventydegrees from horizontal.
 18. The screen assembly as claimed claim 12,wherein said tapered surface is formed from a sheet material.
 19. Thescreen assembly as claimed claim 12, wherein said tapered surface hasopenings therein.